1. Field of the Invention
The present invention relates to methods for improving the quality of meat by reducing boar taint caused by skatole and/or androstenone. The methods involve injecting zinc salts into the testis in a manner that promotes the effectiveness of the injection.
2. Brief Description of the Prior Art
The odor and flavor of pork meat is affected negatively due to the presence of skatole and/or androstenone over certain levels. The odor and flavor may be affected to such an extent that the meat is not acceptable for human consumption.
Skatole and androstenone are unrelated in their production. Androstenone, and other 16 androstenes that are found bound in boar fat, are produced in the testes with pregnenolone as the precursor (Brooks and Pearson, 1989). These compounds are released into the blood via the spermatic vein and are stored in fat (Bonneau and Terqui, 1982). The 5-alpha-androst-16-en-3-one configuration is the predominant storage form because it is the most lipophilic and it is therefore the major component responsible for boar taint (Claus, 1979). During sexual stimulation these steroids can be released back into the bloodstream to travel to the submaxillary gland for use as an important pheromone in saliva (Gower, 1972; Claus, 1979). It is apparent that this storage procedure is also reversible over time because after castration of older males, steroid concentrations in adipose tissue decline (Claus, 1976).
The levels of androstenone found in boar fat vary widely and are affected by age, weight, genetics and stage of sexual maturity (Jonsson and Andresen, 1979; Bonneau, 1981; Willeke, 1980; Walstra, 1984). Additionally, they may be sensitive to external stimuli such as rearing environment or sexual exposure although the literature is somewhat conflicting. As a general rule androstenone production begins increasing slowly when the animal reaches about 70 kg or approximately 4 months of age (Bonneau, 1981). It is also known that the highest incidence of objectionable odor in fat occurs in boars weighing over 95 kg and greater than 5 months of age (Walstra, 1984).
The other component in boar taint is skatole. It is produced by lactobacilli in the hindgut of the pig as these microorganisms break down tryptophan (Yokoyama and Carlson, 1979). Skatole concentration can be altered somewhat by diet and is normally found in excess of threshold levels (0.20 ppm) in boars. However, these levels are rarely reached in barrow or gilt fat (Mortensen et al., 1986) even though gut microflora differs only slightly between the sexes. This may indicate a hormonal influence either in the absorption rate of skatole or the rate of storage and degradation once absorbed (Lundstrom et al., 1988).
Historically, surgical castration has been the choice of most swine producers to deal with boar taint. Surgical castration is commonly carried out by the farmer during the first week of a male pig's life without sedation or anesthetics. The testes of a pig are filled with nerves and, for reasons of animal welfare, it is expected that surgical castration of piglets without anesthesia will be forbidden in the near future, at least in the EU area. In Norway such castration is forbidden from 2009. In the interim period, authorized veterinarians can only perform castration which makes costs prohibitively high for industrial scale pig farming.
Other methods have been tried to overcome the problems associated with surgical castration. For example, intact males have been treated with progestagen either through an implant or in the diet (Berger et al., 1981; Kluber et al., 1988). Immunization of boars against one of the compounds responsible for boar taint has also been tried. (Williamson et al., 1985; Brooks et al., 1986). Both of these methods have serious problems: hormonal treatment must be repeated several times and is not acceptable in swine for human consumption in some cultures. Immunization is not uniformly effective in all animals and there is some risk that cross-reacting antibodies will be induced causing undesired side effects.
In view of the problems with surgical, hormonal and immunocastration, there is a continuing need for an improved method of castration that controls boar taint. The present invention relates to a method of chemical castration and accomplishes those goals.
By way of review, there are two approaches to chemical castration, the first being to inject a sclerotic agent into the vas deferens of an intact male causing an occlusion in the duct blocking the transport of sperm and rendering the animal infertile. This type of chemical sterilization induces infertility but does not reduce the production of testosterone or other testicular steroids responsible for boar taint and male aggressiveness. Many compounds have been shown effective for this purpose: 10 percent silver nitrate or 3 percent formalin (Pineda et al., 1976); 95 percent ethanol (Freeman and Coffee, 1973); dondren (Bierschwal and Ebert, 1961); and quinacrine (Malaviya et al., 1974).
The second type of chemical castration and the type employed in the present invention involves an agent responsible for tissue modification and subsequent disruption of testicular function. The particular effect observed depends on the chemical and concentration injected, species, maturity of the animal and so forth. Background work is described in U.S. Pat. Nos. 4,156,427 and 4,339,438 to Fahim wherein the use of zinc tannate is described and U.S. Pat. No. 5,070,080 to Fahim wherein the use of a neutralized solution of a mineral gluconate and amino acid is described. U.S. Pat. No. 5,372,822 to Fahim describes the use of zinc acetate to control boar taint.
The volume of chemical sterilant that can be injected into a testis is limited because a testis is a closed body. In the above-mentioned patents to Fahim, the chemical sterilant was injected into the midline or bottom of the testis. As mentioned above, the effect on testicular function depends on the chemical nature and concentration of the material injected, hence a method that promotes the effectiveness of the injection such that an effective dose to control boar taint may be delivered in a volume that can be received in the testis is needed.